7. Project Summary The work described in this proposal aims to address the longstanding challenge of controlling the enantioselectivity and regioselectivity of synthetically valuable reactions mediated by radical intermediates. In this work, broadly substrate permissive flavoenzymes are coupled with three novel mechanisms for radical formation to achieve substrate-centered radicals bound within protein active sites. The first mechanism involves electron transfer from a prosthetic flavin cofactor to substrates located within the protein active site. This approach enables the formation of ?-acyl radicals for use in various C?C bond-forming reactions enabling the selective synthesis of carbocyclic and heterocyclic motifs of biological importance. The second involves exploiting the impact that binding has on the redox potential of a chosen substrate. In substrates bearing Lewis basic functional groups, binding within the protein active site makes the substrate easier to reduce. We have found that when weak reductants are used, selective radical formation can be localized to substrates bound to the protein active site. This reactivity enables the formation of ketyl radicals for coupling with electron poor partners such as medicinally valuable heterocycles. Finally, using hydrophobic dyes capable of binding to proteins, we found that radical formation can be selectively localized to enzyme bound substrates. This method enables the selective formation of C?C bonds. Together, these methods and the goals proposed in the Specific Aims have the potential to streamline the synthesis of biological probes and drug targets, creating a significant benefit to human health and associated biomedical sciences. !